The 3875 GC kit guide recommends a 160VA or higher transformer. However, these are really big (toroidals) and I'm building a mono amp with just one 3875 (both channels somehow summed before getting to the 3875) and I want to drive this amp to no higher than 30-40W...maybe rail DC voltage of around 18V or so.

Is it OK to use a smaller toroid? What are the disadvantages of using a smaller toroid...let's say 30 or 60 VA?

Hi,
the big disadvantage of using a small transformer is the high regulation that comes with it.

A 12-0-12Vac transformer is rated at rated current. So 60VA will give 2 times 2.5Arms @ 12Vac.

However on open circuit the voltage will rise by the regulation percentage about (10 to 15% or worse) so the peak voltage at the smoothing caps becomes 12*1.15*1.414=19.5Vpk.
and when the mains voltage rises by another 6% that peak becomes 20.7Vpk.
Take off the diode drop for the rectifier and you have a maximum of 20V on the smoothing caps and that is what your amplifier will feel when idle (your Iq may pull this down slightly).

That high voltage will drop when the amp starts delivering power. This usually shows as poor bass and wandering stereo image, but you only plan one channel so that may not be an issue.

__________________
regards Andrew T.
Sent from my desktop computer using a keyboard

1- The toroids I'm looking at have dual secondary windings. How does this work out with a single LM3875 kit/single rectifier board scenario? Does having dual secondaries help any of the low VA issues brought up?

2- Would going with higher capacity or non-polarized caps help the situation?

Here is some advice for noobs everywhere: when you have a question about an IC or a transformer, the FIRST place you should go is the data sheets and ap notes provided by the manufacturers. You come to places like this only after you have exhausted all other credible sources of information because you can never be sure of the quality of info you'll get here.

Transformer sizing is not a matter of opinion, it is a matter of numbers. The math is easy. The formulas are in the app notes. Can't find an ap note at your transfomer manufacturer's site? Go to another transformer manufacturer's site and use their formula. Power transformers are power transformers. The same formula will apply no matter who makes the thing.

See the capacitor input filter and load near the bottom on the right side? Notice that the DC output current is only 0.62 x the secondary AC current rating. That means that if you need 1 A DC out, your secondary current should be spec'd at 1 A/0.62 = 1.6 Arms.

The DC voltage out is 0.9 x the secondary rms rated voltage. If you want 10VDC, you better have a secondary rated voltage of 10 V/0.9 = 11.1 Vrms.

If you want 10VDC at 1A out, and you don't know better, you calculate that you need 10V x 1 A = 10VA rated transformer, but what you really need is 11.1V x 1.6 A = 18 VA.

You may want to use one of the other configurations shown. Make the appropriate changes to the calcs...

You have to decide what your load is before you calculate the required transformer size. If you were going into production and needed to keep costs down to be competitive, you'd want to be pretty accurate about the calculation so you could use the absolute minimum sized transformer to do the job. Since you are making one unit, and the price difference between a 50VA and a 100VA transformer is pretty small, why not just put in the bigger transformer?

In the above calculation you see that a 10 watt DC load requires about a 18VA transformer. That's about double the DC power. So you can estimate the required power then just double it to size the transformer and you should be in pretty good shape. That will take care of core losses and power dissipated in the rectifiers.